1. Field of the Invention
The present invention relates to a method for producing an image. More particularly, it relates to a novel method for producing a heat resistant, high contrast image having high resolving power using a photographic material which comprises a substrate having thereon at least one silver halide emulsion layer, either directly or on at least one subbing layer on the substrate. It also relates to a novel method for easily producing a durable photomask having high resolving power and good edge acuity using a photographic material which comprises a transparent substrate having a masking layer thereon, the masking layer having thereon at least one silver halide emulsion layer, either directly or on at least one subbing layer on the masking layer.
2. Description of the Prior Art
The optical density of a silver image formed on a photographic material by exposing and developing a photographic material which comprises a silver halide emulsion layer coated on a substrate gradually decreases from its maximum value to a background value at the edges of the silver image. The spacing between the maximum image density portion and the background is usually about 1 micron. Therefore, it is difficult to obtain a high contrast silver image having closely separated, e.g., about one micron, lines or spacings. Silver grains existing between adjacent image lines reduce the image contrast and resolving power.
Moreover, since such an emulsion layer is colored due to thermal decomposition of the binder when it is heated to about 150.degree. to 200.degree. C., it cannot be used where heat resistance is required.
One field which requires heat resistant images is "super-microphotography". An image reduced on a 35 mm film from a 9 by 14 inch (23 by 36 cm) size original with a reduction ratio of about 10 is usually called a "microphotograph", and an image further reduced (to about 2 by 3 mm) by a factor of about 10 is called a "super-microphotograph". A microphotograph can thus be considered to be an image reduced by a factor of about 10 and a super-microphotograph an image reduced by a factor of about 100.
Since the image size of a super-microphotograph is about 2 by 3 mm or smaller, the enlarging factor is about 100 (10,000 based on area ratio) when a super-microphotograph is projected on a screen to provide the original image size. Consequently, a light intensity of about 10 million lux is necessary on the image surface of the super-microphotograph if the image projected on a transmission type screen (e.g., with a blackened back surface) must have a light intensity of about 100 lux when the screen has a transmission optical density of 1. In fact, a super-microphotographic image is illuminated with a light intensity of about 12 to 13 million lux to compensate for the loss of the projection lens.
The temperature of the emulsion layer of a super-microphotograph increases to several hundred degrees C. due to heat generated by the light absorbed in the emulsion layer when a super-microphotograph is continuously illuminated with such strong light. As a result, the binder of the emulsion layer is thermally decomposed and colored, which causes the image projected on the screen to be dim and colored. Since the silver image areas absorb light well, the temperature of these areas preferentially increases and the binder in these areas is preferentially decomposed, whereafter decomposition spreads into surrounding areas. Decomposition of even the binder at non-silver image areas proceeds in an accelerated manner once the binder is slightly colored and light absorption occurs.
It was previously discovered that when an emulsion layer having a silver image therein was heated to about 400.degree. to 600.degree. C. in an oxygen containing gas, such as air, the black silver image turned to a metallic silver image with a mirror surface and the binder of the emulsion layer turned dark red-brown due to thermal decomposition. In a subsequent heating process (hereafter, the heating process is called "baking") in an oxygen containing gas, the binder was decomposed into oxide gases (e.g., CO.sub.2, H.sub.2 O, NO.sub.2 and SO.sub.2) and was removed, and the optical density of the colored binder layer at the non-silver image areas gradually decreased. Also, the mirror surface of the silver image disappeared, probably due to migration of silver particles (probably silver atoms), and at the same time the silver image lost the ability to resolve lines of even about several microns, though the silver image resolved 1 micron lines before baking. It is believed that the silver atoms migrate from their original location to different locations because minute silver crystals are found at the image areas, and the periphery of the image areas, and because many pinholes and cracks occur in the silver image, which decrease the optical density of the baked silver image. In particular, the optical density of high density areas is reduced to a great extent, providing a partially reversed image.
Quite surprisingly, it has been found that decomposed binder remains at non-image areas after decomposed binder at image areas is substantially removed, and that the remaining decomposed binder image has high optical density, smooth edges and high contrast.
Heretofore, emulsion masks and hard masks have generally been used as photomasks for microelectronic manufacture. However, the emulsion mask has low edge contrast as described above and such low mechanical strength that it is easily damaged, that is, durability is poor. On the other hand, a hard mask is quite durable, but processes for the production thereof are complicated. Also, the production of hard masks requires a photoetching process that uses a photoresist which has low speed and requires long exposure times.